Resistors are basic components of most electronic circuits. They are used for biasing, current limiting and a wide range of functions determined by the resistance of the resistor and the particular circuit in which they are used.
Filters are one type circuit employing resistors that are used extensively in the field of electrical signal processing. Fundamentally, the filter has a capacitor and a resistor whose values define frequency response of the filter. Thus, a different value for either the capacitor or the resistor will result in a change in the frequency response.
Typically for prior filters, the frequency response has been changed by substituting a capacitor of one value for a capacitor of another value or by changing a resistor of one value for a resistor of another value. Therefore, a given filter may not be easily used in two different applications requiring different frequency responses since it may be necessary to change either the capacitor or resistor, so that the filter may be usable in either application. Moreover, if the filter is implemented as an integrated circuit, it would not be possible to change the capacitor or resistor.
One application of a filter involves its use in a circuit known as a phase-locked loop. One overall function of a phase-locked loop is to receive an input signal and then output a frequency signal in dependence on any difference in phase between the input signal and the output signal. The phase-locked loop includes a phase detector which receives the input signal and the output signal to generate a phase error signal, the filter that filters the phase error signal and a voltage controlled oscillator that responds to the filtered phase error signal to output and feed back to the phase detector the output frequency signal. The filter will have a frequency response that is predetermined based on the data rate of the input signal that the phase-locked loop is processing.
One problem with a prior phase-locked loop, particularly one being implemented as an integrated circuit, is its inflexibility in being able to be used with different input signals having different data rates. This stems in part from the use of the filter and its fixed frequency response based on the set capacitor and resistor values of the filter.
As the above examples illustrate, the value of resistance of a resistor may have important effects on the operation of circuits employing the resistor.